Important Short Notes

Work & Energy                                           Important Notes

1. Work: Work is said to be done, when a force causes displacement in its own direction.

2. No work is done, if the displacement is not in the direction of applied force or its rectangular component.

3.  Factors which determine work :

(i) Work done is directly proportional to the magnitude of applied force.

(ii) Work done is directly proportional to the displacement in the direction of applied force.

4. Mathematical expression for work :

If F is the force, which causes a displacement S, in its own direction, such that W is the work done, then

W = F × S

5. SI unit of work : SI unit of work is Joule (J).

6.  Bigger units of work :

(a)  kilojoule = 103  J = 1000 J

(b) Megajoule = 106  J = 1000,000 J

(c) Gigajoule = 109  J = 1000,000,000 J

7. Definition of Joule:

When a force of 1 N, causes a displacement of 1 m in its own direction, the work done is said to be one joule.

So, 1 J = 1 N × 1 m = 1 kg ms–2 × 1 m = 1  kg m^2 s^–2.

8. Energy: Capacity of doing work is called energy.

9.  Units of energy: Same as units of work, i.e., Joule.

10. Potential energy: The energy possessed by a body on account of its position or configuration is called potential energy.

11. Mathematical expression for potential energy:

P.E. = mgh

where ‘m’ is the mass, ‘g’ is the acceleration due to gravity and ‘h’ is the height.

12. Characteristics of potential energy:

(a) Potential energy of a body at the surface of earth is taken as zero.

(b) When a body is raised above the ground level, its potential energy increases.

(c) When a body is brought from a height towards the ground, its potential energy decreases.

(d) At any point above the surface of the earth, potential energy is numerically equal to the work done in raising the body.

13. Kinetic energy: The energy possessed by a body by virtue of its motion is called kinetic energy.

14. Mathematical expression for kinetic energy:

K.E.=1/2 m v^2

where ‘m’ is the mass of the body and ‘v’ is the uniform velocity.

15. Power: Rate of doing work is called power.

16. SI unit of power: SI unit of power is watt (W), where 1 W = 1 J/s

17.  Mathematical expression for power:

P = W t

where ‘P’ is the power, ‘W’ is the work done (or energy used) and t is the time in seconds.

18. Bigger units of power :

(a)  kilo watt (kW) = 103 W = 1000 W

(b) Mega watt (MW) = 106 W = 1000,000 W

(c) Giga watt (GW) = 109 W = 1000,000,000 W

19. Definition of watt:  When a work of 1 J is done in 1 s, then the power is said to be 1 watt.

1 W = 1 J / 1 s kg m^2 s^–3.

20. Law of conservation of energy: Energy in a system cannot be created, nor can it be destroyed. It may be transformed from one form to another form, but the total energy of the system remains constant.

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